1. Field of the Invention
The invention relates to a method of producing microstructures having, for example, a different structural height. In particular, the invention relates to a method of producing microstructures having regions of different structural height using a mask, positive x-ray sensitive resist material, and a developer.
2. Background Information
Such a method is disclosed in German Patent No. 3,623,637. In this process, a layer of a positive resist material is partially irradiated with X-ray radiation, with an X-ray mask being employed which is provided with a structured absorber layer that substantially completely absorbs the synchrotron radiation and with at least one further structured absorber layer which absorbs synchrotron radiation preferably only in one part of the spectrum.
A polymer having a sharply defined lower limit dose is employed as the positive resist material.
In the regions not shaded by the structured absorber layer, the resist material, when irradiated, receives a dose higher than the lower limit dose of the resist material along its entire thickness. In the regions shaded by the structured absorber layer, the resist material, when irradiated, receives a dose below the limit dose of the resist material over its entire thickness. In the regions shaded by the structured, partially absorbent absorber layer, a dose greater than the lower limit dose of the resist material is deposited during the irradiation only in the upper portion of the resist material; the lower portion receives a lower dose.
Since the resist material becomes soluble only at the locations that were previously exposed to a dose above the lower limit dose, microstructures are obtained which have regions of different structural height.
This method requires a resist material which has a precisely defined limit dose. On the other hand, the absorber characteristics of the X-ray mask must be carefully adjusted to the resist material.
German Patent No. 3,440,110 discloses another method of the above-mentioned type for the special case of columnar structures having a thin, longer section and a thicker, short section.
In this process, a resist plate of a thickness of about 0.5 mm is partially irradiated through an X-ray mask with high energy X-ray radiation from a synchrotron in such a manner that cylindrical regions result which have a diameter of about 30 .mu.m at a predetermined grid spacing r and whose solubility is much greater than that of the non-irradiated regions of the resist plate. Then the resist plate is again partially irradiated from one side with the same grid spacing r, with, however, the penetration depth of the radiation being less than the thickness of the resist plate and the diameter of the irradiated regions being about 70 .mu.m so that a thicker and shorter cylindrical irradiated region results. The thus irradiated and solubilized regions are removed by means of a liquid developer as disclosed, for example, in German Unexamined Published Patent Application DE-OS 3,039,110. This results in a configuration of columnar structures each having a thinner and a thicker section.
In principle, this method can be transferred to the production of microstructures having other lateral contours and also more than two different structural heights.
One basic problem in the two mentioned methods is that, although in the presently known resist systems there will be no removal during developing if doses below the limit dose are deposited, the mechanical characteristics and the resistance to solvents of these regions are noticeably worse. The height of the structures cannot be predetermined with sufficient accuracy because the weakening of the radiation and thus the dose deposited, is difficult to predetermine with increasing penetration depth.